I do not think that this is possible - You can constrain a drag to the x or y axis but not to a curve. You could try to confine the path in a rectangle object that could be set at a certain angle. In this case you are constraining to a container.

Thanks redesign. What you have pointed me to here and examples in ThreeDubMedia are perfect for what I want to do. I wonder if it would be possible to replace the circle for a square, a zig-zig, or a sine wave?

Sadly, although I can understand the concept of what this code is doing, in practice it is far too complex for me to add to Edge for my epub.

I guess I will have to wait until Adobe make some drag and drop expressions for Edge.